The association of HLA-B27 with ankylosing spondylitis (AS), and other spondyloarthropathies (SpA), remains as one of the strongest between HLA molecules and human disease. Since it was reported, in 1973, it has been extensively studied in order to understand the underlying pathogenic mechanism. The objective of this article is to review the current knowledge on the structure and polymorphism of HLA-B27 molecule, as well as describe the main pathogenic hypotheses trying to explain its association with AS.